Heretofore, communication systems in which a transmission unit (master) and a plurality of terminal devices (slaves) are connected to a transmission channel and communication is performed between the transmission unit and each of the terminal devices are in widespread use. An example of this kind of communication systems is a system in which the transmission unit monitors states of the terminal devices periodically, and when there is a change in the state of one of the terminal devices, the transmission unit transmits a signal to another terminal device so as to perform processing corresponding to the state change (e.g., refer to Japanese Patent Publication No. 1180690, Japanese Patent Publication No. 1195362, and Japanese Patent Publication No. 1144477).
Note that, the communication system having the aforementioned configuration is a system originally used for on-off control of lighting apparatuses or the like, and is not suited to transmitting information having a relatively large data amount such as data represented by an analog quantity, since the communication speed is slow.
In view of this, a communication system that is formed by mixing an existing communication system in which terminal devices communicate with each other via a transmission unit and a communication system in which terminal devices directly communicate with each other by peer-to-peer (P2P) communication has been proposed (e.g., see Japanese Unexamined Patent Application Publication No. 2009-225328). In this communication system, since first terminals (first communication terminals) that communicate via the transmission unit (master) share a communication line (transmission channel) with second terminals (second communication terminals) that communicate with each other directly, the second terminals can be easily added to the existing communication system. The first terminals communicate by a transmission signal (signal conforming to a first protocol) that is repeatedly transmitted to the communication line from the transmission unit, and the second terminals perform higher speed communication by a superimposition signal (signal conforming to a second protocol) that is superimposed on the transmission signal.
Here, the transmission signal is a time-division signal that is divided, frame by frame, into a plurality of regions (periods) in the time axis direction, and some of the regions are assigned as superimposition allowable segments (communication suitable periods) in which the superimposition signal can be superimposed. That is, the second terminals communicate by the superimposition signal that is transmitted on the communication line, on which the transmission signal is also transmitted, in the superimposition allowable segments assigned to part of the transmission signal.
Incidentally, in the communication system described in aforementioned Japanese Unexamined Patent Application Publication No. 2009-225328, although the first terminals and the second terminals share the communication line, communication is performed between the first terminals or between the second terminals independently without interfering with each other, and transmitting and receiving data between a first terminal and a second terminal is not considered. However, in recent years, a flexible system is desired, in which for example an apparatus connected to the second terminal can be controlled based on data input from the first terminal to the transmission unit. In order to achieve this, it is required that transmission and reception of data are performed between the first and second terminals.